1. Field of the Invention
This invention relates to an eye fundus blood flow meter for measuring a blood flow in a blood vessel on the fundus of an eye to be examined.
2. Related Background Art
FIG. 1A of the accompanying drawings shows an example of an eye fundus blood flow meter according to the prior art which is an improvement over a slit lamp generally used for ophthalmic diagnosis and treatment. An illuminating optical system is disposed on an optical path K1, and a white beam of light from an illuminating light source 1 is reflected by an apertured mirror 2 and illuminates a blood vessel Ev on the fundus Ea of an eye E to be examined through a slit 3, a lens 4 and a contact lens 5 which offsets the refractive power of the cornea of the eye E to be examined to thereby enable the fundus Ea of the eye to be observed. A laser light source 6 for measurement emitting He-Ne laser light is disposed on an optical path behind the apertured mirror 2, and the probing beam from the laser light source 6 for measurement passes through the central opening portion of the apertured mirror 2, is made coaxial with the beam of light from the illuminating light source 1 and irradiates the fundus Ea of the eye in the form of a point.
A beam of light scattered and reflected by Red blood cells flowing through the blood vessel Ev and the wall of the blood vessel passes through the objective lenses 7a, 7b of a light detecting optical system for stereoscopic observation disposed on optical paths K2 and K3 forming an angle .alpha.'therebetween, is reflected by mirrors 8a, 8b and mirrors 9a, 9b and is observed as the image of the fundus of the eye by an examiner through eyepieces 10a, 10b, and the examiner selects a measured region while looking into the eyepieces 10a, 10b and observing the fundus Ea of the eye.
FIG. 1B of the accompanying drawings shows the image of the fundus of the eye observed by the examiner. When in an area I being illuminated by the illuminating light, the blood vessel Ev which is the object of measurement is aligned with a scale SC prepared in advance on the focal plane of the eyepieces 10a, 10b, the probing beam from the laser light source 6 for measurement and the blood vessel Ev are aligned with each other, and the measured region is indicated by a spot beam of light PS formed by the laser light source 6 for measurement. At this time, the reflected beam of light of the probing beam by the fundus Ea of the eye is detected by photomultipliers 12a, 12b through optical fibers 11a, 11b.
This detection signal by photomultipliers includes a beat signal component created by a component Doppler-shifted by a blood flow flowing through the blood vessel Ev and a component reflected by the stationary blood vessel wall interfering with each other, and this beat signal is frequency-analyzed to thereby find the speed of the blood flow in the blood vessel Ev.
FIG. 1C of the accompanying drawings shows an example of the result of the frequency analysis of the detection signal by the photomultipliers 12a, 12b, and in this figure, the axis of abscissas represents a frequency .DELTA.f and the axis of ordinates represents the power .DELTA.S thereof. The relation among the maximum shift .DELTA.fmax of the frequency, the wave number vector .kappa.i of the incident beam of light, the wave number vector .kappa.s of the received beam of light and the speed vector .nu. of the blood flow can be expressed as EQU .DELTA.fmax=(.kappa.s-.kappa.i).multidot..nu.. (1)
Accordingly, modifying expression (1) by the use of the shifts .DELTA.fmax1 and .DELTA.fmax2 of the frequency calculated from the respective light detection signals by the photomultipliers 12a and 12b, the wavelength .lambda. of the laser light, the refractive index n of the measured region, the angle .alpha. formed between light detecting optical axes K2 and K3 in the eye and the angle .beta. formed between a plane made by the light detecting optical axes K2 and K3 in the eye and the speed vector .nu. of the blood flow, the maximum speed Vmax of the blood flow can be expressed as EQU Vmax={.lambda./(n.alpha.)}.multidot..vertline..DELTA.fmax1-.DELTA.fmax2.ver tline./cos .beta.. (2)
Thus, by effecting measurement from two directions, the contribution in the direction of incidence of the probing beam is offset, whereby a blood flow in any region on the fundus Ea of the eye can be measured.
Also, to measure the true speed of the blood flow from the relation between the line of intersection A of the plane made by the two light detecting optical paths K2, K3 with the fundus Ea of the eye and the angle .beta. formed between this line of intersection A and the speed vector .nu. of the blood flow, it is necessary to make the line of intersection A coincident with the speed vector .nu. with .beta.=0.degree. in expression (2). Therefore, in the example of the prior art, the entire light detecting optical system is rotated or an image rotator is disposed in the light receiving optical system, thereby making the line of intersection A optically coincident with the speed vector .nu..
In the above-described example of the prior art, however, the maximum value .DELTA.fmax of the Doppler shift is detected as the interference signal between the component shifted by the blood flow and the stationary blood vessel wall and thus, the maximum shift .DELTA.fmax obtained by frequency analysis becomes .vertline..DELTA.fmax.vertline. which lacks sign information.
Thus, when measuring the blood flows in blood vessels in different regions of the fundus Ea of the eye, there are cases where the signs of the maximum frequency shifts .DELTA.fmax1 and .DELTA.fmax2 both have the positive sign, both have the negative sign, and have the positive and the negative sign, respectively. Accordingly, this gives a rise to a problem that depending on the area to be measured, it becomes impossible to determine the maximum blood flow speed Vmax by expression (2).
This problem will now be described by the use of FIG. 1D of the accompanying drawings. When in FIG. 1D, signal light enters from the center hi=0 of the pupil Ep and scattered light is received from the predetermined regions hs1 and hs2 of the pupil Ep, the angle at which the predetermined regions hs1 and hs2 are subtended from the fundus Ea of the eye is the angle .alpha. formed between the light detecting optical axes in the example of the prior art shown in FIG. 1A.
Considering now a case where a blood vessel Ev1 at the center of the fundus Ea of the eye and a blood vessel Ev2 in the marginal region of the fundus Ea of the eye are to be measured, when the measurement of the blood vessel Ev1 is effected, the maximum frequency shift .DELTA.fmax1 obtained by the light reception signal from the direction of the region hs1 and the maximum frequency shift .DELTA.fmax2 obtained by the light detection signal from the direction of the region hs2 assume different signs. In this case, the signal light is incident on the blood vessel Ev1 perpendicularly thereto and thus, the frequency shift caused by the direction of the signal light is null and the frequency shift obtained is only caused by the direction of observation. Considering here the speed vector .upsilon. of the blood flow in the blood vessel Ev1, the wave number vector .kappa.s1 in the direction of hs1 and the wave number vector .kappa.s2 in the direction of hs2, these exist in different directions relative to the perpendicular to the speed vector .upsilon. and therefore, the inner product thereof assumes a different sign and frequency shifts of different signs occur.
On the other hand, when the measurement of the blood vessel Ev2 in the marginal region is effected, the direction of hsl and the direction of hs2 exist in the same direction relative to positive reflected light .kappa.' whose frequency shift is 0 and thus, frequency shifts of the same sign occur. Here, the angle formed between a straight line linking the center Eo of the fundus Ea of the eye and the blood vessel Ev2, i.e., the perpendicular at the blood vessel Ev2 of the fundus Ea of the eye, and the direction of the wave number vector .kappa.i of the signal light is .phi.i, and a wave number vector indicative of the positive reflected light of the vector .kappa.i being at an angle .phi.c with respect to the perpendicular and facing in opposite direction to the vector .kappa.i is .kappa.i'.